1. Field of the Invention
This invention relates to testing of electrical distribution circuits, and particularly to a tester that plugs into a receptacle and tests especially arc fault protection, but can also perform ground fault, grounded neutral and proper wiring tests.
2. Background Information
The common type of circuit breaker used for residential, commercial, and light industrial applications has an electro-mechanical thermal-magnetic trip device to provide an instantaneous trip in response to a short circuit and a delayed trip in response to persistent overcurrent conditions. Some such circuit breakers include ground fault protection which trips the circuit breaker in response to a line to ground fault, and in some cases, a neutral to ground fault. Ground fault protection is provided by an electronic circuit which is set to trip at about 4-6 ma of ground fault current for people protection, and at about 30 ma for equipment protection. It is known to incorporate a test circuit in the circuit breaker which tests at least portions of the electronic ground fault test circuit. It is also known to test for proper wiring connections. Test circuits for this purpose are commercially available.
More recently, interest has arisen in providing protection against arc faults. Arc faults are intermittent, high impedance faults which can be caused for instance by worn insulation, loose connections, broken conductors and the like. Arc faults can occur in the permanent wiring, at receptacles, or more likely, in the wiring of loads or extension cords plugged into the receptacle. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device which provides the short circuit and overcurrent protection.
Arc fault detectors are generally of two types. One type responds to the random high frequency content of the current waveform generated by an arc. The other basic type of arc fault detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc fault detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006 and 5,691,869. Built-in test circuits have also been proposed for such arc fault detectors. U.S. Pat. No. 5,459,630 discloses several forms of built-in test circuits for such arc fault detectors. In one embodiment, in which the arc fault detector utilizes a coil to sense current, the test circuit adds a capacitor which forms with the impedance of the coil an oscillator generating a waveform with an amplitude which simulates the rapid rise of a step change in current produced by an arc. In another embodiment, the user must repetitively close a switch which connects a resistor between the line conductor and neutral to again generate large amplitude pulses.
While the built-in arc fault and ground fault testers test the response of the electronic circuits to simulated conditions, they do not necessarily indicate whether the device will adequately respond in a real installation. One difficulty is that the circuit breaker containing the detectors is located at a load center together with the circuit breakers for other the circuits in the installation. However, the fault condition can occur anywhere downstream and can be further distanced from the circuit breaker and detectors by an extension cord. The wiring, and particularly the extension cord, can insert considerable resistance between the fault and the detector which attenuates the signal sensed by the detector. When the effects of this resistance are combined with the low amplitude of the currents generated by these faults, the detectors may not have sufficient sensitivity to detect remote faults. Another problem can be a receptacle that is not connected as intended.
There is a need therefore for improved test circuits for electrical distribution systems and especially for testing arc fault detectors.
There is also a need for apparatus for testing arc fault and ground fault detectors for a response to faults in actual installations, especially faults which are remote from the detectors. In other words, there is a need for testers which verify whether protection is actually being provided at a remote location in a distribution circuit.
There is also a need for such testers which are flexible, simple, and economical.